A recently developed type of secondary or rechargeable electrical conversion device comprises: (A) an anodic reaction zone containing a molten alkali metal anode-reactant, e.g., sodium, in electrical contact with an external cirucit; (B) a cathodic reaction zone containing (i) a cathodic reactant comprising sulfur or a mixture of sulfur and molten polysulfide, which is electrochemically reversibly reactive with said anodic reactant, and (ii) a conductive electrode which is at least partially immersed in said cathodic reactant; and (C) a solid electrolyte comprising a cation-permeable barrier to mass liquid transfer interposed between and in contact with said anodic and cathodic reaction zones. As used herein the term "reactant" is intended to mean both reactants and reaction products.
During the discharge cycle of such a device, molten alkali metal atoms such as sodium surrender an electron to an external circuit and the resulting cation passes through the solid electrolyte barrier and into the liquid electrolyte to unite with polysulfide ions. The polysulfide ions are formed by charge transfer on the surface of the electrode by reaction of the cathodic reactant with electrons conducted through the electrode from the external circuit. Because the ionic conductivity of the liquid electrolyte is less than the electronic conductivity of the electrode material, it is desirable during discharge that both electrons and sulfur be applied to and distributed along the surface of the conductive material in the vicinity of the cation-permeable solid electrolyte. When the sulfur and electrons are so supplied, polysulfide ions can be formed near the solid electrolyte and the alkali metal cations can pass out of the solid electrolyte and the alkali metal cations can pass out of the solid electrolyte into the liquid electrolyte and combine to form alkali metal polysulfide near the solid electrolyte.
During the charge cycle of such a device when a negative potential larger than the open circuit cell voltage is applied to the anode the opposite process occurs. Thus electrons are removed from the alkali metal polysulfide by charge transfer at the surface of the electrode and are conducted through the electrode material to the external circuit, and the alkali metal cation is conducted through the liquid electrolyte and solid electrolyte to the anode where it accepts an electron from the external circuit. Because of the aforementioned relative conductivities of the ionic and electronic phases, this charging process occurs preferentially in the vicinity of the solid electrolyte and leaves behind molten elemental sulfur. As can be readily appreciated the production of large amounts of sulfur near the surface of the cation-permeable membrane has a limiting effect on rechargeability. This is the case since sulfur is nonconductive and when it covers surfaces of the electrode, charge transfer is inhibited and the charging process is greatly hindered or terminated. Thus, in order to improve the rechargeability of a cell of this type it is necessary not only to supply polysulfide to the surface of the porous electrode in the vicinity of the cation-permeable membrane, but also to remove sulfur therefrom.
U.S. Pat. No. 3,811,493 and U.S. Pat. No. 3,980,496 both disclose energy conversion device designs which allow or promote improved mass transportation or reactants and reaction products to and from the vicinity of the solid electrolyte and the electrode during both discharge and charge. In the device disclosed in U.S. Pat. No. 3,811,493 an ionically conductive solid electrolyte is located between a first reactant in one container and a second reactant in another container. An electrode for one of the reactants comprises a layer of porous, electronically conductive material having one surface in contact with one side of the ionically conductive solid electrolyte and the other surface in contact with a structurally integral electronically conductive member permeable to mass flow of its reactant and electrically connected to the external circuit. An open volume exists between the structurally integral conductive member and the container wall to promote free flow and mixing of the reactant. Reactants also flow readily through the conductive member into the layer of porous electronically conductive material. The conductive member distributes electrons to the porous, conductive material which in turn transfers electrons to or from the reactants.
The improvement disclosed in U.S. Pat. No. 3,980,496 comprises designing the cathodic reaction zone of the device such that there are a plurality of channels and/or spaces within said zone which are free of porous conductive electrodes and which are thus adapted to allow free flow of the molten cathodic reactants during operation of the device. This flow results from free convection within the channels and/or spaces, and from wicking of cathodic reactants within the conductive porous material.
U.S. Pat. No. 3,976,503 discloses an improved method for recharging secondary batteries or cells of the above-described type. The process involves maintaining a temperature gradient within the cathodic reaction zone during recharging such that the temperature of the cathodic reactants in a first region adjacent the solid electrolyte or cation-permeable barrier is sufficiently higher than the temperature of said reactants in a second region not adjacent the barrier such that sulfur in the first region vaporizes and is transported to said second region where it condenses.
U.S. Pat. No. 3,966,493 discloses an improved secondary battery or cell of the type described above which exhibits increased ampere-hour capacity as the result of an improvement which comprises: (a) employing a porous conductive material which will wick both sulfur and alkali metal polysulfides and which, in different regions of said cathodic reaction zone exhibits different degrees of wettability by said alkali metal polysulfides, and said material in a region adjacent to said cation-permeable barrier being more readily wetted by said polysulfides than is said material in a region further removed from said barrier such that sulfur will boil near said barrier and condense away from it; (b) disposing said porous conductive material within said cathoidc reaction zone such that it forms and encloses one or more channels which extend from said region adjacent said cation-permeable barrier outwardly into said region of said cathodic reaction zone which is further removed from said barrier; and (c) maintaining the amount of molten cathodic reactant within said cathodic reaction zone such that said channels remain free of said molten reactant and are thus adapted to transport sulfur vapor.
U.S. Pat. No. 3,951,689 discloses still another improved secondary battery or cell of the type described above which exhibits increased ampere-hour capacity as the result of an improvement which comprises: adapting the cathodic reaction zone to operate as a gas fuel cell electrode by employing a sulfur storage chamber containing molten sulfur connected with said cathodic reaction zone so as to allow sulfur vapors to pass therebetween, the storage chamber being adapted to be maintained at a temperature (i) above the temperature of said cathodic reaction zone when said cell is being discharged such that sulfur distills into said cathodic reaction zone and (ii) below the temperature of said storage chamber.
The devices of U.S. Pat. Nos. 3,966,492 and 3,951,689 each employ electrode materials which are preferentially wet by polysulfide salts as is the case in the invention covered by this application. However, unlike the invention of this application, each also teaches the use of such a material in conjunction with an electrode material which is preferentially wet by sulfur. Also each of those devices, unlike the device of this application, relies on vapor transport for the removal of objectionable sulfur from the region of the electrode near the cation-permeable barrier. The process of U.S. Pat. No. 3,976,503 also relies on vapor transfer of sulfur and requires the maintaining of a temperature gradient using external heating elements.
U.S. Pat. No. 4,002,806 teaches increasing the ampere-hour capacity of a secondary battery or cell of the type described by including certain metals, metal salts and other metal compounds in the cathodic reactant. The mechanism by which these materials increase ampere-hour capacity of the device is not known. One of several theories mentioned in the application is that the materials may to an extend coat the graphite felt, thereby rendering it preferentially wettable by polysulfide and, thus, increasing charge efficiency. Such a mechanism, as stated therein, is only one of several possibilities, and is not certain. It is just as likely that, when these materials are dissolved in or mixed with the polysulfide melt they impart general or localized electronic conductivity to the melt, thereby extending the effective electrode area, altering the electrode kinetics and improving charge capacity. A still further theory suggests that the materials disperse in the melt as a solid phase and thereby increase effective electrode area so as to increase capacity. In any event, the battery or cell disclosed in U.S. Pat. No. 4,002,806 unlike the battery or cell made in accordance with the improvement of this invention, requires the addition of materials to the cathodic reactant.
The prior art designs disclosed and claimed in U.S. Pat. Nos. 3,811,493 and 3,980,496 are effective in promoting distribution of reactants during both discharge and charge. However, even with these improved designs it is difficult to recharge the batteries or cells at high rates.
It has been found that by employing the improvement of this invention, which may be combined effectively with the improvements of U.S. Pat. No. 3,980,496, it is possible to obtain a cell which, without the necessity of external heating or cooling or other modifications, exhibits a high efficiency on charging, thus increasing the ampere-hour capacity of the battery or cell.